In numerous applications such as laser tracking, laser guidance and laser imaging, it is desirable to produce a high power output laser beam to illuminate uniformly a large target area. Moreover, high power laser systems find applications in such diverse fields as offensive and defensive weapon systems, e.g., non-visible light illuminators for special operation forces, for example gunship camera illuminators, and civilian protective laser grids for high value assets, such as nuclear power plants and airports. In short, there are multiple military and commercial applications for high power laser sources to uniformly illuminate target areas. In the earliest laser systems, single flashlamp pumped solid state lasers were utilized to provide a source of laser output for illuminating extended target areas, but these were limited in the amount of power they could provide due to their structural limitations and limited efficiency, nor was the illumination of the target area sufficiently uniform for many purposes. Subsequently, arrays of semiconductor lasers have been utilized in which the beams from adjacent emitters of the array, in both 2D rack-and-stack configurations or spaced upon the same substrate, were coupled together in the same optical aperture to uniformly illuminate a target area. The illumination can be for various purposes, for example, rendering objects of interest visible in the illuminated area to a camera system or making range measurements to points on the illuminated surface by operating the laser in short pulses.
More recently, fiber optic power amplifiers have been employed to produce a high-power output signal. While a single fiber power amplifier will suffice for some low power applications, an array of optical fiber amplifiers collectively forming the fiber optic laser source can be employed in those specific applications when higher power output laser beams are required. Furthermore, the use of pulsed fiber lasers is highly desirable for space based 3-D imaging and target classification, the more so if the high efficiency, flexible waveform characteristics, beam quality and compact packaging of the fiber source can be maintained. Unfortunately, the pulse energy that can be achieved from a single fiber amplifier is not adequate for many important applications, thus requiring some method for combining the outputs from a multiplicity of fiber devices. While several approaches for scaling have been investigated, include spectral and coherent beam combining, all have potential problems for application requiring uniform illumination of a target area. For example, in an important Light Amplification for Detection and Ranging (LADAR) application, it is desired that an array of lasers transmit and scan time synchronized pulse beams at a same wavelength to uniformly cover an area in the far field and capture the return signals on a two-dimensional detector. However, interference can occur between beams at small regions of overlap potentially producing noise and degrading performance.